Generally, it is preferred to drill arcuate paths or tunnels beneath water courses, roadways, and other surface obstacles, especially where there is a need to forego dredging a river bed, digging a trench, or otherwise altering the obstacle. For example, when a road is encountered, the road must either be closed, the traffic diverted, or parts of the road closed and then conduit laid in section, all of which are inconvenient and interfere substantially with the use of the road. Likewise, problems arise when a conduit must span a water course. First, the trench must be dug to a depth considerably below the level at which the conduit is to be laid since the water current will cause partial filling of the trench before the conduit can be inserted. This is a very significant problem because such trenches have a generally triangular cross-section and the volume of dirt which must be removed increases with the square of the depth of the trench. Moreover, such trenching stirs the alluvium at the bottom of the water course interfering with the natural flora.
Even after a trench has ben dug in a water course, difficulties arise in placing the conduit therein. One method of placing the conduit is to float it across the span of the water course and then remove its buoyancy to sink the pipe into the trench. The difficulty with this method is that the water course must be closed to traffic, floating objects are trapped by the floating conduit, the current of the river bends the conduit, and when the conduit is sunk, it quite often misses the trench. The second method is to attach a sled or skid to the leading edge of the conduit, and drag this leading end through the trench to lay the conduit. With this method, the conduit must be coated with the substance to give it negative buoyancy, and this coating is quite expensive since the entire length of the conduit must be so coated.
Traditional methods of digging wells cannot be adapted to digging arcuate paths such as that required for implanting a pipeline or other conduit under an obstacle. Such traditional methods utilize a drill stem which enters the ground substantially normal to its surface. If these methods were utilized, the drill stem would have to undergo a 180 degree turn to span the obstacle. Such a path would result in a drill string extending vertically upwardly at the other side of the obstacle with the weight of the drill string acting against the forward motion of the drill. Since the weight of the string is used to drive the drill, such a method cannot be used when drilling the upward portion of an inverted arcuate path. Moreover, traditional well drilling methods rely on the weight of the pendant drill string to achieve a substantially vertical hole. In the past, when an angular bend in the hole was desired, it was the practice to lower a whip stock shim into the bottom of the hole which shim forced the drill off at an angle. Later methods utilized a self-powered drill which had a drill stem slightly angularly inclined with respect to the drill string. The problem with both of these methods is that the entire drill string must be removed from the hole, either to lower the whip stock or to mount the inclined drill. After the angle has been made, the entire drill string must be withdrawn again to remove either the whip stock or the inclined drill. Hence, these methods have been used primarily for drilling controlled arcuate paths and are useful for intermittent bends and/or constant radius turns.
Techniques have been developed for drilling holes along a substantially linear horizontal path for placing telephone lines under streets and the like. However, these methods employ drills which proceed in a straight line, and to achieve an arcuate path, a pothole must be dug and the drill manually redirected. Such a method is often acceptable in traversing an obstruction such as a road, particularly a road which is constructed such that it has high shoulders on both sides, but is impractical for traversing a water course because of the expense involved in digging the required potholes.
Each of the aforementioned methods utilizes a traditional drill comprising a rotary bit which bit mechanically erodes the earth with which it comes into contact. The eroded earth is then carried away by a continuously moving stream of fluid passing from the surface through the bore or drill stem to the rotating bit, through the bit and back through the space between the drill string and the hole wall, and then back to the surface thus carrying away the debris. These methods are simplified through utilization of a jet bit wherein the jet bit utilizes a plurality of jet nozzles to direct a pressurized fluid spray against the earth both to erode the earth and to supply the fluid with which to transport and otherwise facilitate removal of the debris produced by action of the jet spray.